The invention relates in general to apparatus for supplying controlled direct current power to a load and more particularly to an improved circuit of the chopper-type using solid state controlled rectifiers. The use of SCR devices in DC to DC power converters is known: see for instance "Principles Of Inverter Circuits" B. D. Bedford and R. G. Hoft -- John Wiley 1964, pages 340-357. Chopper circuits are used for the purpose of controlling the flow of power from a DC source to a load, which may be a DC motor. One important element of a chopper circuit is the commutating circuit. While SCR devices are used to switch a load current in the power path from the DC source to the load, control and regulation require controlled conduction of the main power switch. When gated the SCR device is turned on, however, a commutating circuit is necessary to turn off again the conducting SCR device. Generally, a commutating circuit comprises a commutating capacitor and a commutating thyristor. Some problems arise for an efficient use of the commutating capacitor. For instance, it is known to use an autotransformer in association with the commutating capacitor to insure that the capacitor is always charged up whenever load current starts to flow (see General Electric Silicon Controlled Rectifier Manual, 1964, page 169, FIG. 9.2.11 "The Jones Chopper").
Still the circuits of the prior art are not always satisfactory from the point of view of reliability, efficiency, or cost.
An object of the present invention is to provide a circuit for controlling DC power which is stable in operation, capable of increasing the upper limit for the frequency range of operation, of broadening the range of control of the DC power for a given size of the commutation capacitor and of reducing the size of the commutation capacitor for a given range of power control.